1. Field of the Invention
The present invention relates to a tachogenerator for speed control of electric motors and in particular for brushless d.c. motors.
2. Summary of the Invention
The present invention provides a tachogenerator for speed control of electric motors having an axial bearing for compensating the axial forces acting on the motor. A flat multipole permanent magnet is solidly connected to the rotor of the motor and has a number p of pairs of magnetic poles on its surface uniformly arranged on a circle of diameter d for the pole centers. A circular wave winding is solidly connected to the stator and separated from the permanent magnet by a flat air gap of a width g and has a number p of angular substantially repeat units u of the wave winding. The number of pole pairs p follows the formula EQU p=(0.5 to 1.5).multidot.(d/g)
wherein d is the diameter of the centers of the poles, g is the width of the air gap and d and g are measured in the same units.
Preferably the number p of pole pairs is equal to from about 0.8 to 1 times the ratio of the diameter d divided by the width g, with d and g again being measured in the same units. Preferably the diameter d is at least about 10 mm and at most about 100 mm and more preferred is a range of from about 30 mm to 50 mm. Preferably the width g of the air gap is at least about 0.2 mm and at most about 2 mm and more preferred is a range for g of from about 0.5 to 1 mm.
Preferably the number p of pole pairs of the multipole permanent magnet is equal to from about 1 to 2 times the number p of pole pairs corresponding to the maximum induced voltage at the frequency desired.
It can be advantageous for the tachogenerator to comprise in addition to the wave winding a corresponding return winding running in opposite direction to the wave winding for compensating the effect of the wave winding on a magnetic field component in axial direction running through the circle of the wave winding. The wave winding can be a meander winding. The meander winding can be provided by a conductor disposed on a plastic disc by photolithographic techniques.
Also, a soft magnetic material preferably in magnetic shield form can be disposed on the side of the wave winding opposite to the side facing the multipole permanent magnet.
The wave winding is preferably attached to a support tube of the rotor. The multipole permanent magnet is preferably attached to a soft magnetic backing solidly connected to the rotor. The multipole permanent magnet can be a so called rubber magnet and this rubber magnet can be adhesively attached to a soft magnetic material. A preferred electric motor is a two phase brushless d.c. motor which can comprise successively a north pole magnet zone of 120.degree. electrical, a south pole magnet zone of 120.degree. electrical and a remaining zone of 120.degree. electrical having a magnetization which generates in a magnetically active section of the stator winding at a relative motion a substantially zero voltage.
In a preferred embodiment of the invention a brushless d.c. motor with a flat air gap is provided wth a tachogenerator for speed control. At least one stator coil is disposed for generating an electromagnetic torque and a rotor magnet arrangement is disposed at a certain distance from the stator coil around a rotation axis. The rotor magnet arrangement is preferably an axially polarized magnet ring. An axial bearing is provided for compensating the axial forces exerted on the rotor, especially as disclosed in German patent application No. P2,730,142.4-32 or, respectively, U.S. Pat. No. 4,211,963. In the space between the rotation axis and the rotor magnet arrangement there is disposed a flat multipole permanent magnet and connected to the rotor of the motor in a solid angular relation.
Opposite to the rotor and separated by an air gap which is at least nearly parallel to the air gap of the motor is disposed a wave or meander winding at the stator. A voltage is induced in the wave or meander winding upon operation of the motor by the flat multipole permanent magnet and the frequency of the voltage is proportional to the speed of the motor.
The air gap of the motor on the side opposite to the side facing the rotor magnet can be surrounded by soft ferromagnetic materials for increasing magnetic field strength and these soft ferromagnetic materials and the stator are provided with a feedthrough for the connectors of the wave or meander winding.
Preferably the number p of pole pairs of the multipole flat permanent magnet and the number of the magnetically active sections of the wave or meander winding are selected such that in a diagram, showing the number of pole pairs on the abscissa and the voltage u induced in the wave or meander winding at a constant speed of rotation with a constant frequency n, the number p of pole pairs is in the region of the voltage maximum and preferably above said maximum.
The air gap of the motor can be bordered on the side away from the rotor magnet by stationary soft magnetic materials and these soft magnetic materials can be formed as plates, they can be pierced in their middle by a bearing tube and they can be together with the bearing tube surrounded by plastic.
The plastic can also support the stator winding and at the same time provide insulation.
The size of the maximum of the voltage u depends on the frequency, but does not shift with frequency. Preferred materials for the multipole magnet are hard magnetic materials with high coercitive force such as plastic barium ferrite, sintered barium ferrite or samarium cobalt. When the magnetic pole material demagnetizes, then a lower pole pair number is preferred. By the arrangement of the tachogenerator in the space between the rotation axis and the rotor magnet there results a compact, relatively small arrangement wherein stray fields from the stator winding or from the rotor are less effective in this region as in the air gap of the motor and if desired the stray fields can also be screened by iron sheets or the like in the usual fashion.
Since by experience the axial bearing of the rotor sinks somewhat in with increasing age and therefor the air gap of the tachogenerator decreases, the output voltage increases with increasing age of the motor resulting in an improvement of the uniform running properties with increasing age.